1. Field of the Invention
The present invention relates to an inkjet printhead element substrate which comprises a printing element, a shift register, a latch circuit and a delay circuit which delays an input signal and outputs the signal, and a printhead, head cartridge, and printing apparatus using the element substrate.
2. Description of the Related Art
In a recent printhead used for an inkjet printing method (liquid-jet printing method), thermal energy generated by heaters serving as printing elements is applied to a liquid to cause it to bubble. The energy for generating a bubble causes orifices to discharge ink droplets. Such a printhead has a number of very small heaters arranged on a silicon semiconductor substrate to enable high-density printing. Additionally, orifices are arranged to oppose each of the heaters. Driving circuits for driving the heaters and other logic circuits are also provided on the silicon semiconductor substrate. For example, several tens to several thousands of heaters, drivers for driving the respective heaters, a shift register having bits as many as the heaters, and a latch circuit for temporarily storing print data (print signal) output from the shift register are provided on a single silicon semiconductor substrate. Note that the shift register finally sends the serially input print data to the drivers in parallel.
That is, integration of drivers and logic circuits such as a shift register and latch circuit on an element substrate has progressed recently. In this case, a current that flows to one heater instantaneously reaches a considerably large value. If a lot of heaters are turned on simultaneously, a pulse-like current of, for example, about 1 to several A flows to the power supply line and ground (GND) line for driving the heaters.
When a current flows, induction noise is generated by inductive coupling in flexible wirings from the printing apparatus main body to the printhead or wiring in the printhead. If a pulse-like current having a large value flows, as described above, operation errors may occur in the logic circuit parts on the printhead element substrate. Unwanted electromagnetic noise may be radiated externally.
Induction noise more readily occurs, and its noise level rises as the current change amount per unit time increases. More specifically, when the number of orifices provided in the printhead increases, and the number of elements turned on simultaneously increases for high-speed or high-resolution printing, the value of the pulse-like current also becomes large, resulting in higher noise level.
To prevent this, the orifices are divided into a plurality of blocks, and the blocks are time-divisionally driven, instead of simultaneously driving many heaters provided on the printhead element substrate. More specifically, at a given timing, the first block is selected to drive the heaters while inhibiting driving of the heaters in the remaining unselected blocks. At the next timing, the heaters in the second block are selectively driven while inhibiting driving of the heaters in the remaining blocks. All blocks are selected in this way one after another, thereby completing one cycle of driving of the heaters corresponding to all orifices.
However, if many orifices exist (if many heaters exist), the number of orifices per block also increases. For this reason, the current value does not sufficiently decrease so it is impossible to suppress the amount of induction noise generation. If the number of blocks is increased to reduce the number of heaters to be turned on simultaneously, the time allotted to every block shortens. Hence, it may be unable to obtain sufficient energy for ink discharge. To obtain desired energy, the time allotted to every block is made long. However, this reduces the printing speed.
There is disclosed an arrangement which shifts, little by little, the driving pulse to be applied to heaters belonging to a single block (Japanese Patent Publication Laid-Open No. 07-68761). More specifically, in forming an element substrate for an inkjet printhead, a hysteresis circuit is provided in an input unit together with the elements of heaters, drivers, and logic discharge control circuits such as a shift register. To apply driving pulses to different heaters at different timings, a CR (capacitor-resistor) integrating circuit is formed in the signal path of a heat pulse signal (input pulse width signal) that defines the pulse width and timing of a driving pulse. The heat pulse signal is delayed to sequentially drive the heaters. That is, the current flowing to the heaters is controlled by shifting the timing of the heat pulse signal using the CR integrating circuit. This reduces the number of heaters to be turned on at the same timing and decreases the peak value and rise ratio of the current generated by the driving pulse, thereby suppressing noise. Even when the number of heaters to be driven simultaneously increases due to an increase in the number of orifices or high-density arrangement of orifices necessary for high-speed printing, induction noise generation is suppressed.
However, even in the arrangement that prevents noise by using the CR integrating circuit, as disclosed in Japanese Patent Laid-Open No. 07-68761, if C (capacitor) and R (resistor) vary, the product of them generates a variation in the delay value of the heat pulse signal. It is therefore impossible to accurately control the current flowing to the heaters. It may consequently be unable to sufficiently suppress noise. The CR integrating circuit includes an input buffer, capacitor, and resistor. If the wiring length difference between these logic circuits becomes large, the delay value varies. In an inkjet printhead element substrate which is manufactured by using a typical silicon semiconductor device manufacturing technology, often, a capacitor uses a gate oxide film, and a resistor uses a diffused resistor. For this reason, if a CR integrating circuit having a desired time constant is formed, the capacitor and resistor occupy a large area of the element substrate for an inkjet printhead, resulting in a bulky inkjet printhead element substrate.
There is proposed an arrangement which forms, on an input line to input a pulse width defining signal, a CMOS inverter circuit serving as a logic circuit for applying a driving pulse to heaters at different timings (Japanese Patent Laid-Open No. 2004-50846).
On the other hand, the recent inkjet printhead element substrate is positively introducing high-density integration of heaters and an increase in the number of nozzles in order to improve the printing speed and image quality.
As described above, high-density integration of heaters and an increase in the number of nozzles are positively introduced recently. The high-density integration of heaters can be achieved by reducing the ink droplet size and arranging nozzles at a high density. However, to maintain the same printing speed, the driving frequency needs to be higher than before along with the increase in the integration density. In addition, to obtain a higher printing speed than before, the driving frequency needs to be further higher.
When the driving frequency rises, the driving period shortens naturally. In this case, when the heat pulse signal is delayed by using a delay circuit in correspondence with the number of heaters in a block from the viewpoint of noise reduction, as described above, the heat pulse signal may exceed the span of the latch signal period from a latch signal to the next latch signal depending on the delay amount. If the heat pulse signal exceeds the span of the latch signal period, when the heat pulse signal is being input to the heaters in a given block, the logic may switch to drive different heaters halfway so the desired heaters cannot be driven. Hence, there is a demand for development of an element substrate capable of stably driving desired heaters even when the driving frequency rises.
The above-described problem arises not only when the driving frequency rises but also when the number of heaters to be driven in a single block increases.
The technique disclosed in Japanese Patent Laid-Open No. 2004-50846 can surely suppress noise and prevent an increase in the size of an inkjet printhead element substrate. However, it cannot solve the problem that driving of desired heaters is hindered at a higher driving frequency.